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Avalanche Forecast

Archived

Jan 20th, 2020–Jan 21st, 2020

Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches possible, human triggered probable.
Alpine
Natural avalanches possible, human triggered probable.
Treeline
Natural avalanches possible, human triggered probable.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural and human triggered avalanches likely.
Treeline
Natural and human triggered avalanches likely.
Below Treeline
Natural avalanches possible, human triggered probable.

Regions

Northwest Coastal.

Storm totals in the south of the region are in the range of 60-90 cm. The new snow rests on a mix of weak facets and old wind slabs. Continue to make conservative terrain choices and avoid overhead hazard as the snowpack adjusts to the new load.

Confidence

Moderate - Uncertainty is due to the limited number of field observations. Uncertainty is due to the speed, direction, or duration of the wind and its effect on the snowpack.

Weather Forecast

Monday night: Clearing, flurries tapering to trace amounts.Moderate wind switching southwest to southeast. Alpine low -4 C. Freezing level around 800 m.

Tuesday: Increasing cloud with isolated flurries bringing up to 10 cm of new snow. Light wind from the south to southeast. Alpine high -3 C. Freezing level around 600 m.

Wednesday: Mostly cloudy with isolated flurries bringing 5-10 cm of snow. Strong southwest winds. Alpine high -3 C. Freezing level around 600 m.

Thursday: Mostly cloudy with heavy snowfall bringing 30-50 cm of snow, concentrated in the south of the region. Strong southwest winds, approaching extreme at ridgetop. Alpine high -3 C. Freezing level around 1000 m.

Avalanche Summary

A few natural and skier triggered storm slab avalanches up to size 1.5 were reported over the weekend. Avalanche observations have likely been hindered by limited visibility through the storm, so we expect details of storm slab activity to come in after it clears up.

Reports of deep persistent slab avalanches have been trickling in throughout the month. They are associated with a November crust layer near the base of the snowpack which has produced very large avalanches (size 3+) with crown depths of around 2 m. Observations are typically in alpine terrain, on lee or cross-loaded slopes. 

Snowpack Summary

Storm totals in the south of the region are in the range of 60-90 cm. Windslabs formed in the alpine may now be hidden by newly fallen snow due to strong southerly winds dying down towards the tail of the storm. 

The new snow rests on extensively wind affected surfaces at upper elevations, and a thick layer of weak facets in sheltered areas treeline and below.

A deep crust from mid November lurks at the base of the snowpack. A couple of recent large avalanches have run on this layer on lee and cross-loaded features high in the alpine. As this storm applies significant new snow loads to the snowpack, these deep layers will feel the stress and we may see some very large avalanches in high places.

Terrain and Travel

  • Continue to make conservative terrain choices while the storm snow settles and stabilizes.
  • Don't let the desire for deep powder pull you into high consequence terrain.
  • Avoid freshly wind loaded terrain features.
  • Be aware of the potential for large avalanches due to the presence of a deep persistent slab.

Problems

Storm Slabs

Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.